The present invention relates to a process for forming foamed fibers, and especially to processes employing a combination of molten polymer containing therein a dissolved decomposable compound or gaseous blowing agent and a closed-cell-forming additive which is extruded and subsequently quenched under conditions to produce an improved foamed fiber. The present invention also includes novel foamed fibers having essentially only closed-cell bubbles contained therein, and novel foamed fibers having substantially uniform cross sectional area cells formed therein.
Foamed thermoplastic (and especially polyamide) fibers have been produced, especially for the purpose of being broken (fibrillated) into three-dimensional structures of interrelated fiber elements. See, for example, U.K. Patent Specifications Nos. 1,316,465, 1,221,488, 1,296,710, and 1,318,964. In addition, foamed polyester and polyamide fibers for textile applications are disclosed in DOS No. 2,148,588 (Apr. 5, 1973) (See Example 7). See also Chem. Abstract 90:24692m (1979) of Japanese Kokai No. 78,106,770.
Hollow fibers, also known in the art, contain elongated voids extending generally or the entire length of the fiber in the longitudinal direction. Some of these fibers contain large diameter voids with low total void volume and find use in thermal insulation. The elongated voids are generally produced by the use of a modified spinning die.
U.S. application Ser. No. 490,070, entitled "Producing Foamed Fibers, " to H. L. Li et al., filed Apr. 29, 1983 and commonly assigned now U.S. Pat. No. 4,562,022, discloses improved methods of forming fine-celled foamed fibers which employs at least one additional member arranged above the spinnerette which, with extruding a polymer melt having a blowing agent admixed therewith, produces excellent foamed fiber products.
We have discovered a method of forming foamed fibers which contain fine, closed-cell bubbles, and/or cells of uniform cross sectional area. To that end, we have discovered a class of additives (hereinafter referred to as closed-cell-forming additives) which, when included in the polymer melt, produce improved foamed fibers. The use of the additive in a process for foaming fibers also dramatically enhances the ability to draw the fibers to produce very fine (on the order of 1 dpf) fibers (comprising open and/or closed cells) which are particularly useful as, for example, filter material, acoustic insulation, and apparel fiber.